This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Appln. No. 199 57 981.4 filed in Germany on Dec. 2. 1999; the entire content of which is hereby incorporated by reference.
The present invention relates to the field of high-temperature superconductors. It relates to a high-temperature superconductor arrangement and to a method for its production.
When high-temperature superconductors are used for high electrical power levels, the problem of so-called hot spots must be addressed. The critical current density is not constant throughout the entire superconductor, owing to unavoidable material inhomogeneities in the superconductor, or because of local thermal fluctuations. In consequence, for example as a result of a short circuit, an initial rise in the fault current will result in the current density exceeding the local critical current density at the weakest point of the superconductor first of all. A voltage drop thus starts to build up at this point in the superconductor. In the process, resistive heat is produced, which heats the superconductor in a small area, and causes the superconductivity to break down locally. A hot spot is produced which, in the end, leads to destruction of the superconductor.
First aid is offered by an electrical bypass which makes electrical contact with a high-temperature superconductor over its entire length, and is thus connected in parallel with any potential hot spot. The electrical bypass represents an alternative current path by means of which the short-circuit current can bypass the hot spot, thus homogenizing the voltage distribution.
In order to protect the superconductor against hot spots efficiently, the bypass layer must have a certain minimum conductivity. However, if the superconductor is used in a current limiter a specific minimum total bypass resistance is required in order to ensure the required limiting characteristics. The bypass resistance per unit length must therefore not be too small, in order to prevent the conductor length of the current limiter from increasing excessively. The bypass must therefore not conduct too well, or must have an appropriately small cross section.
EP 0 911 889 A2 is based on the object of preventing hot spots in current limiters for electrical power levels of more than 1 MW. The arrangement proposed for this purpose comprises a superconductor layer which makes contact with two normally conductive bypass layers via a main surface. The first bypass layer, which is not made of noble metal, is composed of steel and is at the same time used as a heat sink. The second bypass layer, which contains noble metal, is composed of silver or a silver alloy, onto which the steel layer is soldered. In order to avoid the total bypass resistance from being too small, the second bypass layer must be as thin as possible, or its resistivity must be increased by adding gallium or lead to the alloy.
Furthermore, DE-A 196 34 424 A1 discloses a method for producing a composite conductor composed of a high-temperature superconductor and a bypass layer. A silver foil is in this case applied to an inert powder layer and is covered by a so-called green sheet, which contains an oxide-ceramic powder as the base material for the high-temperature superconductor, and an organic binding agent. The layers are then fusion-processed, that is to say once the binding agent has been burnt out, the powder is partially melted. In the process, the silver is partially dissolved in the liquid high-temperature superconductor, and the thickness of the silver layer is reduced in an uncontrolled manner. This results in excellent contact resistance between the silver layer and the superconductor layer, of less than 1 xcexcxcexa9cm2 at 77 K. At least one layer composed of a fiber-composite material is also applied, to provide mechanical strength.
The object of the present invention, in a high-temperature superconductor arrangement of the type mentioned initially, is to improve the contact between the electrical bypass and the superconductor without in the process significantly increasing the conductivity of the arrangement. This object is achieved by a high-temperature superconductor arrangement having the features of patent claim 1, and by a method for its production having the features of patent claim 8.
The essence of the invention is to couple the electrical bypass to the superconductor via a contact-making layer which is distinguished by anisotropic conductivity which is particularly pronounced in a direction at right angles to the main current flow direction, or at right angles to the conductors in the arrangement. This allows the current to commutate from the superconductor into the bypass, without the total resistance of the bypass and contact-making layer being reduced.
According to a first preferred embodiment, the anisotropy of the conductivity is achieved by the contact-making layer not being continuous, that is to say it is interrupted by discontinuities running transversely with respect to the main current flow direction and is broken down, for example, into a number of strips.
According to a further embodiment, the contact making layer is a more or less uniform distribution of individual islands which are not connected to one another.
The said two embodiments have the advantage that the thickness of the contact-making layer does not influence its conductivity in the main current flow direction, and it can thus be composed of a very good and isotropic conductor. In consequence, it is possible to choose silver or gold as materials for the contact-making layer, which materials can form a low-resistance contact with the superconductor in a sintering or heat-treatment process, without adversely affecting its characteristics.
In a third embodiment, a further, continuous intermediate layer is provided between the contact-making layer and the electrical bypass.
The advantages of the method according to the invention are that the preparation of the contact-making layer is separated from the process of applying the electrical bypass. The co-sintering or post-sintering processes which are suitable for achieving a good contact resistance between the superconductor and the contact-making layer are carried out in advance and do not adversely affect the electrical characteristics of the bypass.
Further advantageous embodiments are evident from the dependent patent claims.